From related art, an electric supercharger equipped with an electric motor that adds rotational driving force to a rotary shaft connected to a compressor wheel in a supercharger has been known (see, for example, Patent Literature 1). The electric motor mounted on a supercharger described in Patent Literature 1 includes a motor rotor (a rotor) fixed to the rotary shaft. The motor rotor includes an inner sleeve mounted on the rotary shaft, a permanent magnet which surrounds the inner sleeve around an axis, an end ring (a ring member) disposed on the outside of the permanent magnet in an axial direction of the inner sleeve and shrink-fitted to an outer periphery of the inner sleeve.